This invention relates to a catalytic electrode, particularly an electrode with a nano-catalytic material, an electrochemical cell containing the catalytic electrode, and processes for making the catalytic electrode and the electrochemical cell.
There is a wide variety of electrochemical cells that have catalytic electrodes. Examples include, but are not limited to, fuel cells, metal-air battery cells, gas (e.g., hydrogen) generating cells, and electrochemical sensor cells. Examples of such cells are found in U.S. Pat. Nos. 5,242,565; 5,308,711; 5,378,562; 5,567,538; 5,707,499; 6,060,196; 6,461,761; 6,602,629; 6,911,278; 7,001,689 and 7,001,865; and in International Patent Publication No. WO 00/36677.
An advantage of cells with catalytic electrodes is that they can use one or more active materials that are not contained within cell or battery housings, thereby providing long use time (e.g., discharge capacity) with a cell having a minimum volume. There is an ongoing desire to improve the performance of such electrochemical cells, such as by improving electrical characteristics (e.g., operating voltage, power output, energy density, discharge capacity, charging efficiency, cycle life and fade), storage characteristics, leakage resistance, cost, environmental impact of waste disposal, and safety in manufacturing.
One way to improve the electrical characteristics is through the use of a catalytic material having greater catalytic activity. One approach to doing so has been to use nano-catalysts (catalyst materials with particles smaller than one micrometer (μm) because their large surface area provides more reactive sites. To provide good electrical conductivity and low internal resistance within the catalytic electrode and to provide a large reactive surface area with the electrode, the catalytic material is often combined with a porous material with excellent electrical conductivity. Examples of such efforts can be found in U.S. Pat. No. 7,087,341; U.S. Patent Publication No. 2006/0226564; U.S. Patent Publication No. 2006/0269823; U.S. Patent Publication No. 2007/0092784; U.S. patent application Ser. No. 11/482,290, filed Jul. 7, 2006; and Unexamined Japanese Patent Publication No. 2006-252,797.
Because of the high reactivity of nano-catalyst materials, it may be necessary to process the nano-catalyst material in an inert environment, such as an Argon atmosphere. This places restrictions on and adds cost to the manufacturing process that can be undesirable, especially for large scale, high speed manufacturing. Previous efforts to control the reactivity of nano-particulate catalyst compositions include processes in which the substrate particles (e.g., an activated carbon) and nano-catalyst particles (e.g., a nano-metal) are treated in a liquid medium (e.g., a lower alcohol such as methanol), and the nano-particles are oxidized by removing the liquid medium and contacting the nano-particles with an oxidant.
Previous efforts have not been completely successful in producing catalytic electrodes and electrochemical cells using catalytic electrodes that have improved performance characteristics and are easy, economical and safe to manufacture.